Detection of underground objects has long been a very active research subject because of its important applications in mine detection, geological exploration, forensic investigation, treasure hunting, etc. Recent emphasis on environmental clean-up and remediation, has increased interest in this area. In converting abandoned military bases to civil use and in cleaning old battle fields, objects left in the ground, such as shells, unexploded ordnance (UXO), etc., need to be detected and removed.
Most of the techniques and systems currently available for the detection of underground objects are of the type which search the designated area and indicate the possible existence of some underground objects. They lack the capability to precisely locate and characterize the objects.
The difficulty in characterizing underground objects with electromagnetic (EM) methods stems from the ground medium surrounding the objects. Unlike air which is transparent and almost lossless for EM wave propagation, the ground is conductive and generally inhomogeneous causing an increase in signal dissipation and localization errors. Since dissipation increases with the EM frequency, it limits the use of the EM energy to low frequency bands when depth penetration beyond several meters is desired.
In air, the electromagnetic field satisfies the Helmholtz wave equation and operates in a propagation mode at distances between the source and the receiver much greater than a free space wavelength. The amplitude of the fields either remains constant or changes relatively slowly, but the phase changes rapidly with distance. From the phase delay of an electromagnetic field reflected from a target one can tell the range between the target and the observer. The directivity, or focused direction, along which the antenna transmits and receives the electromagnetic field gives the direction of the target. The range combined with the direction indicates the location of the target.
In contrast, in the ground, at measurement ranges much less than a wavelength, the electromagnetic field basically satisfies a diffusion type equation in the low frequency limit, rather than the usual wave equation as in the air. For distances much less than a wavelength, the phase delay, related to the spatial variation of the waves (i.e., ei2xcfx80x/xcex), is too small to be used to measure the object""s range and shape or to resolve multiple objects. In addition, the small size of the transmitter and receiver aperture relative to a wavelength does not provide a capability for measuring direction to the buried object.
Because of the potential danger involved in the remediation of abandoned military bases and old battlefields, not just the detection of the underground objects is required, but a more precise characterization which includes location, orientation, size, shape, and material composition is desirable. New technological systems which can satisfy these requirements are in urgent demand.
The invention is a method of localizing highly conducting, (e.g., metallic) buried objects by measuring their field distribution on or above the surface and then reconstructing the field as a function of depth in the ground utilizing a nearfield holography algorithm. The three dimensional locations of the objects are determined from the reconstructed field images of the objects. Also disclosed is a time-domain electromagnetic sensor system that collects the data used by the nearfield holographic technique of the invention.
The conventional ranging method is based on the observation of a phase change, i.e., the temporal change, of the field at a fixed station in space. For underground objects located at a distance of a small fraction of a wavelength, i.e., located in the nearfield, the temporal change of the field is negligible. However, in the diffusion region the magnitude of the field is very sensitive to the distance from the object. Since the field decays approximately inversely as the third power of the distance, a small change in distance leads to a significant change in the field magnitude. Based on this fact, instead of observing the temporal field change at a fixed point in space, more accurate information about underground objects can be obtained by examining the spatial change of the field in a plane above the ground.
In its simplest form, the nearfield holographic technique is a combination of an EM measurement procedure and a method for treating the resulting EM data. The area where the object is buried is illuminated with an active transient time-domain EM source located on or above the ground and scanned over an mxn grid on the surface. The field radiated from the source penetrates into the ground and induces eddy currents inside the object. These eddy currents act as a secondary source which re-radiates EM fields. The re-radiated EM fields from the object, or the time-rate-of-change of the secondary magnetic fields, are then measured at each of the points on the mxn grid in a horizontal plane at the surface of the ground in the vicinity of the buried object.
For a sensor/receiver coil operating in a time domain mode, the received signal at each grid point is Fourier transformed to the frequency domain, so that the secondary magnetic fields re-radiated from the object are obtained as a function of frequency. The characteristic frequency from the resulting frequency domain response, or the measurement frequency for a sensor/receiver coil operating in the frequency domain, is used as the image reconstruction frequency.
The magnetic field, after being transformed into the frequency domain, is a complex function having both magnitude and phase. At a particular frequency, the magnetic fields at the individual grid points in the detection plane form a spatial distribution of the measured magnetic field in that plane. The spatial variation of the magnetic field depends on the characteristics of the buried object and the distance from the object. This spatial distribution of the magnetic field at the detection plane is used to reconstruct the magnetic field distribution in the horizontal plane at various depths in the ground by spatial Fourier transforms and a backward propagation algorithm. It is first Fourier transformed in two dimensions to form the spatial frequency spectrum. This spectrum is multiplied by a propagation function determined from the Helmholtz equation for a desired depth, d. The result is inverse Fourier transformed in two dimensions to produce the reconstructed image at depth, d.
The above process is repeated for a range of depths, d, that bracket the expected depths of the buried objects. The resulting images in the back propagated planes, i.e., the reconstructed magnetic field distributions, are then examined to resolve closely spaced objects and to determine the depth and the location in the horizontal plane. The depth of the object is the point where the image is the smallest, or focused, since the re-radiated fields expand in all directions from the location of the object.
Thus, the invention meets the goal of locating and resolving highly conducting buried objects such as UXO and mines, which are small in size and shallow compared to general geological structures, that are buried in the nearfield, i.e., at depths and separated by distances much less than a wavelength.